In closure units of this type, the clamping plates or the molding tool halves are driven apart and together and the actual closing force and the opening force is applied with the same hydraulic cylinders. The hydraulic system must therefore inevitably be considerably over-dimensioned so that large quantities of hydraulic fluid have to be conveyed at high pressure in a correspondingly time-consuming manner.
In a different, known closure unit a movable cross head which can be locked on the columns by clamping action is also provided. For raising and lowering this cross head with upper clamping plate and upper molding tool half there are provided between the fixed and the movable cross head some long lifting cylinders whose power merely needs to be adapted to the weight of the contacting parts to be moved. Their size is correspondingly small. Short lifting cylinders are provided between the movable cross head and the upper clamping plate for applying the closing force and for opening the molding tool once the molding has cured, using so-called movable beams. They have the advantage that their hydraulic chambers can be kept relatively small so that they are less complex in terms of design and operate more rapidly owing to the smaller quantities of hydraulic fluid. As closure units are also generally equipped with pivot devices, the additional spatial requirements through these short lifting cylinders is disadvantageous. The required synchronization is not always ensured during the opening process either, so that damage to the molding and, in the long run, also damage to the dividing planes of the molding tool halves can occur due to tilting of the upper half of the molding tool relative to the lower half of the molding tool.
The object is to provide a closure unit of the type mentioned at the outset in which the necessary force systems for the various functions are simple and compact in design and operate rapidly and reliably.